This invention relates to a method and apparatus for controlling a printer and, more particularly, to such a method and apparatus for insuring that an indicium of uniform intensity is printed by a respective print head in response to data representing a predetermined data level regardless of the number of print heads included in the printer which are energized concurrently.
In a so-called line printer, a linear array of print heads is provided, each being selectively energized to print, or record, an indicium. Such print heads may be thermal print heads, wherein an indicium, or mark, is produced in response to thermal energy generated by that print head. As an alternative, the print heads may be photographic-type, wherein a mark is produced in response to light energy generated thereby. In both types of print heads, the intensity, or "blackness" of the mark, is a function of the duration that the print head is energized. For example, a darker mark is produced by a thermal print head when that head is energized for a longer period of time.
In the aforementioned line printer formed of a linear array of thermal print heads, data representing the intensity, or darkness of the mark to be produced by each print head is represented by a respective signal. Each such signal may be a digital signal, wherein the value of that digital signal represents the level of intensity of the mark which is to be printed. For example, data formed of a 4-bit digital signal is capable of representing any one of sixteen different intensity levels. The data signal [0000] may represent the lightest mark to be printed, and the data signal [1111] may represent the darkest mark to be printed. Thus, a print head which is energized in response to the data signal [0000] is energized for the least duration; and the print head which is energized in response to the data signal [1111] is energized for the maximum duration.
A a result of the selective energization of the line of print heads, a corresponding line of optically viewable information is printed. This information may be in the form of alphanumeric characters, a graphical representation, or a picture representing a scene, or other information. After one complete line of indicia, or marks, is printed, the recording medium, which may be conventional paper, specially treated paper, thermally responsive paper, photographically responsive film, or the like, is advanced so that the next-following line may be printed. Hence, successive lines of marks, or "dots", of differing intensity, result in the formation of a viewable image having perceptible contrasts.
A typical line printer is provided with groups of print heads, each group being formed of a number (e.g. 64) of print heads. As a numerical example, twenty groups, each consisting of 64 print heads, may be provided, resulting in a line printer formed of 1,280 print heads. For convenience, such a line printer is supplied with groups of data signals, each group serving to selectively energize a corresponding group of print heads. Consistent with the foregoing numerical example, in a line printer formed of twenty groups of print heads, twenty groups of data signals are supplied, each group being formed of 64 data signals. After the first group of print heads is energized in response to a corresponding group of data signals, the next group of print heads is energized, and so on, until the entire line of marks, or indicia, is printed.
However, it has been found that the energy which is supplied to each print head in a group of print heads varies as a function of the total number of print heads which are to be energized. That is, when thermal print heads are used, the current flowing through a particular one of such thermal print heads is less when all of the print heads in that group are energized concurrently than if that particular print head is the only one in the group which is energized. Consequently, with the same data signal used to energize that print head, a mark of lesser intensity is formed thereby when all of the print heads in the group are energized than when only that print head in the group is energized. Typically, more print heads in a group will be energized to represent lighter marks; and a lesser number of print heads will be energized to form marks of higher intensity levels. This means that, when the concentration of printed marks should be reduced, as when the intensity levels of the printed marks are high, the printed marks actually will be darker than expected because higher currents flow through the energized print heads when a smaller number of such heads is energized. As a result of this higher concentration of smaller numbers of marks, the resultant picture quality is perceived as being unnatural.